In response to cellular stresses including, but not limited to heat, toxins, radiation, infection, inflammation, and oxidants, all cells produce a common set of heat shock proteins (Hsps) (Macario & de Macario 2000). Most heat shock proteins act as molecular chaperones. Chaperones bind and stabilize proteins at intermediate stages of folding and allow proteins to fold to their functional states. Hsp90 is the most abundant cytosolic Hsp under normal conditions. There are two human isoforms of Hsp90, a major inducible form Hsp90α and minor constitutively expressed form Hsp90β and two other closely related chaperones which are restricted in their intracellular location (Endoplasmic reticulum GP96/GRP94; mitochondrial TRAP1). The term HSP90 as used here includes all these analogues unless stated. Hsp90 binds proteins at a late stage of folding and is distinguished from other Hsps in that most of its protein substrates are involved in signal transduction. Hsp90 has a distinct ATP binding site, including a Bergerat fold characteristic of bacterial gyrase, topoisomerases and histidine kinases. It has been shown that ATP bound at the N-terminal pocket of Hsp90 is hydrolysed. This ATPase activity results in a conformational change in Hsp90 that is required to enable conformational changes in the client protein.
A dimerization domain and a second ATP binding site, which may regulate ATPase activity, is found near the c-terminus of Hsp90. Dimerization of HSP90 appears critical for ATP hydrolysis. Activation of Hsp90 is further regulated through interactions with a variety of other chaperone proteins and can be isolated in complex with other chaperones including, but not limited to Hsp70, Hip, Hop, p23, and p50cdc37. Many other co-chaperone proteins have also been demonstrated to bind HSP90. A simplified model has emerged in which ATP binding to the amino terminal pocket alters Hsp90 conformation to allow association with a multichaperone complex. First the client protein is bound to an Hsp70/Hsp40 complex. This complex then associates with Hsp90 via Hop. When ADP is replaced by ATP, the conformation of Hsp90 is altered, Hop and Hsp70 are released and a different set of co-chaperones is recruited including p50cdc37 and p23. ATP hydrolysis results in the release of these co-chaperones and the client protein from the mature complex. Ansamycin antibiotics herbimycin, geldanamycin (GA) and 17-allylamino-17-desmethoxygeldanamycin (17-AAG) are ATP binding site inhibitors that block the binding of ATP and prevent conversion to the mature complex (Grenert et. al., 1997. J Biol. Chem., 272:23834-23850).
Despite Hsp90 being ubiquitously expressed, GA has a higher binding affinity for Hsp90 derived from tumour vs. normal cell lines (Kamal et. al., Nature 2003; 425: 407-410). GA also shows more potent cytotoxic activity in tumour cells and is sequestered at higher concentrations within tumours in xenograft mouse models (Brazidec J. Med. Chem. 2004, 47, 3865-3873). Furthermore the ATP-ase activity of Hsp90 is elevated in cancer cells and is an indication of the increased level of stress in these cells. Hsp90 gene amplification has also been reported to occur in the later stages of cancer (Jolly and Morimoto JNCI Vol. 92, No. 19, 1564-1572, 2000).
Increased genetic instability associated with the cancer phenotype leads to an increase in the production of non-native or mutant proteins. The ubiquitin pathway also serves to protect the cell from non-native or misfolded proteins, by targeting these proteins for proteasomal degradation. Mutant proteins are by their nature not native and therefore have the potential to show structural instability and an increased requirement for the chaperone system. (Giannini et al., Mol Cell Biol. 2004; 24(13):5667-76).
There is some evidence that Hsp90 is found primarily within “activated” multichaperone complexes in the tumour cells as opposed to “latent” complexes in normal cells. One component of the multichaperone complex is the cdc37 co-chaperone. Cdc37 binds Hsp90 at the base of the ATP binding site and could affect the off rates of inhibitors bound to Hsp90 in the “activated” state (Roe et. al., Cell 116, (2004), pp. 87-98). The client protein bound to the Hsp90-Hsp70 form of the chaperone complex is believed to be more susceptible to ubiquitination and targeting to the proteasome for degradation. E3 ubiquitin ligases have been identified with chaperone interacting motifs and one of these (CHIP) was shown to promote the ubiquitination and degradation of Hsp90 client proteins (Connell et al., 2001. Xu et al., 2002).
Hsp90 Client Proteins
The number of reported Hsp90 client proteins now exceeds 100. Since many of its client proteins are involved in cell signalling proliferation and survival, Hsp90 has received major interest as an oncology target. Two groups of client proteins, cell signalling protein kinases and transcription factors, in particular suggest Hsp90 regulation may have potential benefit as an anticancer therapy.
Hsp90 protein kinase client proteins implicated in cell proliferation and survival include the following:
c-Src
Cellular Src (c-Src) is a receptor tyrosine kinase, required for mitogenesis initiated by multiple growth factor receptors, including, but not limited to the receptors for epidermal growth factor receptor (EGFR), platelet-derived growth factor receptor (PDGFR), colony stimulating factor-1 (CSF-1R), and the basic fibroblast growth factor (bFGFR). C-Src is also overexpressed and activated in many of the same human carcinomas that overexpress EGFR and ErbB2. Src is also required for the maintenance of normal bone homeostasis through its regulation of osteoclast function.
p185erbB2
ErbB2 (Her2/neu) is a receptor tyrosine kinase overexpressed in a variety of malignancies including, but not limited to breast, ovarian, prostate, and gastric cancers. ErbB2 was originally identified as an oncogene and inhibition of Hsp90 results in the polyubiquitination and degradation of erbB2.
Polo Mitotic Kinase
Polo-like kinases (Plks) are important regulators of cell cycle progression during M-phase. Plks are involved in the assembly of the mitotic spindle apparatus and in the activation of CDK/cyclin complexes. Plk1 regulates tyrosine dephosphorylation of CDKs through phosphorylation and activation of Cdc25C. CDK1 activation in turn leads to spindle formation and entry into M phase.
Akt (PKB)
Akt is involved in pathways that regulate cell growth by stimulating cell proliferation and suppressing apoptosis. Hsp90 inhibition by ansamycins results in a reduction in the Akt half life through ubiquitination and proteasomal degradation. Binding of cdc37 to Hsp90 is also required for the down-regulation of Akt. Following ansamycin treatment cancer cells arrest in the G2/M phase of the cell cycle 24 hours after treatment and proceed to apoptosis 24-48 hours later. Normal cells also arrest 24 hours after ansamycin treatment, but do not proceed on to apoptosis.
c-Raf, B-RAF, Mek
The RAS-RAF-MEK-ERK-MAP kinase pathway mediates cellular responses to growth signals. RAS is mutated to an oncogenic form in approximately 15% of human cancers. The three RAF genes are serine/threonine kinases that are regulated by binding RAS.
EGFR
The epidermal growth factor receptor (EGFR) is implicated in cell growth, differentiation, proliferation, survival, apoptosis, and migration. Overexpression of EGFR has been found in many different cancers and activating mutations of its kinase domain appear to be pathogenic in a subset of adenocarcinoams of the lung.
Flt3
FMS-like tyrosine kinase 3 (FLT3) is a receptor tyrosine kinase involved in cell proliferation, differentiation and apoptosis. Flt3 activation also leads to the activation of phosphatidylinositol 3-kinase (PI3K) and RAS signal-transduction cascades.
c-Met
c-met is a receptor tyrosine kinase which binds hepatocyte growth factor (HGF) and regulates both cell motility and cell growth. c-met is overexpressed in tumours, including, but not limited to thyroid, stomach, pancreatic and colon cancer. HGF is also detected around the tumours, including liver metastases. This suggests that c-met and HGF play an important role in invasion and metastasis.
Cdk1, Cdk2, Cdk4, Cdk6
Cdk1, Cdk2, Cdk4, and Cdk6 drive the cell cycle. The activity of CDKs is regulated by their binding to specific subunits such as cyclins, inhibitory and assembly factors. The substrate specificity and timing of CDK activities is dictated by their interaction with specific cyclins. Cdk4/cyclin D and Cdk6/cyclin D are active in the G1 phase, Cdk2/cyclin E and Cdk2/cyclin A in S phase, and Cdc2/cyclin A and Cdc2/cyclin B in G2/M phase.
Cyclin-dependent kinase type 4 (CDK4), plays a key role in allowing cells to traverse G1 to S-phase transition of the cell cycle and is constitutively activated in many human cancers. The CDK4 activator, cyclin D1, is overexpressed and a CDK4 inhibitor, p16, is deleted in a variety of human tumours.
Cdk1/Cdk2 inhibitors have been developed which reversibly block normal cells in either the G1/S-phase or at the G2/M border. G2/M arrest is generally less well tolerated by the cells and consequently, they undergo apoptotic cell death. Since Hsp90 also is known to affect cell survival pathways this effect may be further amplified with an Hsp90 inhibitor.
Wee-1
The Wee-1 protein kinase carries out the inhibitory phosphorylation of CDC2 on tyrosine 15 (Tyr15). This is required for activation of the G2-phase checkpoint in response to DNA damage.
Hsp90 transcription factors implicated in cell proliferation and survival include the following:
Mutant p53
P53 is a tumour suppressor protein that causes cell cycle arrest and induces apoptosis. P53 is mutated in approximately half of all cancers. Mutant p53 associates with Hsp90 and is down-regulated in cancer lines treated with Hsp90 inhibitors, while wild type p53 levels were unaffected.
Estrogen Receptor/Androgen Receptor
Approximately 70% of post-menopausal women who develop breast cancer have tumours that express the estrogen receptor. The first line treatment of these patients is directed at preventing signalling through this pathway and thus inhibiting tumour growth. This can be done by ovarian ablation, treatment with gonadotrophin releasing hormone agonists, aromatase inhibition or treatment with specific agonists which bind to the estrogen receptor but prevent further signalling. Ultimately patients develop resistance to these interventions often as a consequence of crosstalk between the estrogen receptor and growth factor receptors located on the cell membrane. In the unliganded state estrogen receptors are complexed with Hsp90 which facilitates hormone binding. Following binding to the mature receptor Hsp90 complex the liganded receptor can bind to hormone-response elements (HREs) within the regulatory regions of target genes involved in maintaining cell proliferation. Inhibition of Hsp90 initiates proteosomal degradation of the estrogen receptor thus preventing further growth signalling via this pathway. Prostate cancers are hormone-dependent malignancies that respond to therapeutic interventions which reduce circulating levels of testosterone or prevent testosterone binding to the androgen receptor. Although patients initially respond to these treatments most subsequently develop resistance via restoration of signalling via the androgen receptor. Prior to ligand binding the androgen receptor exists in a complex with Hsp90 and other co-chaperones including p23 and immunophilins. This interaction maintains the androgen receptor in a high-affinity ligand binding conformation. Inhibition of Hsp90 leads to proteosomal degradation of the androgen receptor and other co-chaperones which may sensitise the tumour to further hormonal therapies.
Mutated steroid hormone receptors that have arisen for example during anti-hormone therapy and which might be resistant to such therapies are likely to have a greater dependence on HSP90 for their stability and hormone binding function.
Hif-1a
Hypoxia inducible factor-1a (HIF-1a) is a transcription factor that controls the expression of genes which play a role in angiogenesis. HIF-1a is expressed in the majority of metastases and is known to associate with Hsp90. Ansamycin treatment of renal carcinoma cell lines leads to the ubiquitination and proteasomal degradation of HIF-1a. Hsp90 inhibitors are capable of affecting a large number of targets significant to signal transduction in tumour cell proliferation. Signal transduction inhibitors which regulate the activities of a single target, may not be as efficacious due to signalling pathway redundancy and the rapid development of resistance.
By regulating multiple targets involved in cell signalling and cell proliferation HSP90 inhibitors may prove beneficial in the treatment of a wide spectrum of proliferative disorders.
ZAP70
ZAP-70, a member of the Syk-ZAP-70 protein tyrosine kinase family, is normally expressed in T cells and natural killer cells and has a critical role in the initiation of T-cell signaling. However, it is also expressed aberrantly in approximately 50% of cases of CLL, usually in those cases with unmutated B-cell receptor genes. The mutational status of immunoglobulin heavy-chain variable-region (IgVH) genes in the leukemic cells of chronic lymphocytic leukemia (CLL) is an important prognostic factor. The expression of ZAP-70 in CLL cells correlates with IgVH mutational status, disease progression, and survival. ZAP-70 positive CLL is more aggressive than ZAP-70 negative CLL indicating that ZAP-70 may be a key driver of malignancy in this disease. ZAP-70 is physically associated with HSP90 in B-CLL lymphoblasts thus the inhibition of Hsp90 may sensitise these cells to existing chemotherapy or monoclonal antibody therapy.
HSP90 Inhibitors as Anti-Fungal, Anti-Protozoal and Anti-Parasitic Agents
Fungal infections have become a major cause for concern in recent years because of the widespread use of immunosuppressive therapies and the increasing incidence of species that are resistant to established antifungal agents such as the azoles. The growing population of immunocompromised patients (e.g. patients such as organ transplant patients, cancer patients undergoing chemotherapy, burn patients, AIDS patients, or patients with diabetic ketoacidosis) has given rise to an increase in the incidence of opportunistic fungal infections by fungal agents such as Candida, Cryptoccocus and Aspergillus species and, on occasion, Fusarium, Trichosporon and Dreschlera species. 
Consequently, there is a need for new anti-fungal agents that can be used to treat the growing numbers of patients with fungal infections and in particular infections due to fungi that have become resistant to existing antifungal drugs.
HSP90 is conserved across evolution being found in bacteria (e.g. HTPG in E. coli) and yeast (e.g. HSC82 and HSP82). Although clients have not been formally identified for the E. coli form, in yeast and all higher organisms the HSP90 family has been shown to function as a chaperone for many essential proteins as described above.
Infection by a range of pathogens is associated with an antibody response to HSP90. For example in Candida albicans infected patients the 47 kDa C-terminal fragment of HSP90 is an immunodominant epitope. Furthermore this antibody response is associated with good prognosis suggesting a protective effect against infection. Recombinant antibodies to an epitope in this polypeptide are also protective against infection in mouse models of invasive candidiasis. (See Mathews et al Antimicrobial Agents and Chemotherapy 2003 vol 47, 2208-2216 and references therein). Likewise surface expressed HSP90 serves as an antigen in Chagas' disease, ascariasis, leishmaniasis, toxoplamosis and infection due to Schistosoma mansoni and it has been postulated that antibodies to HSP90 convey protection against plamodium infection and Malaria.
Mycograb (NeuTec Pharma/Novartis) is a human recombinant monoclonal antibody against heat shock protein 90 that is being developed as a treatment for candida and has shown significant responses in early trials. Furthermore, the natural product HSP90 inhibitors Geldanamycin, Herbimycin and Radicicol were originally identified by their anti-fungal activity. Key essential proteins have been identified as HSP90 clients in several human pathogens (see Cowen and Lindquist, Science. 2005 Sep. 30; 309(5744):2175-6.) Thus HSP90 can play an important role in the growth of pathogens such as Candida species, and HSP90 inhibitors can be useful as treatments for a range of infectious diseases including candidiasis.
It has also been found that Hsp90 increases the capacity of fungi to develop antifungal drug resistance (see Cowen L E, Lindquist S. “Hsp90 potentiates the rapid evolution of new traits: drug resistance in diverse fungi”. Science. 2005 Sep. 30; 309 (5744):2185-9). Therefore, co-administration of an Hsp90 inhibitor with an antifungal drug may enhance the efficacy of the antifungal drug and reduce resistance by preventing the emergence of resistant phenotypes.
HSP90 Inhibitors in the Treatment of Pain, Neuropathic Conditions and Stroke
Cdk5 is a member of the Cdk family of serine/threonine kinases, most of which are key regulators of the cell cycle. Cdk5 activity is regulated through association with its neuron-specific activators, p35 and p39. Recent evidence suggests that CDK5 can phosphorylate tau protein and a number of other neuronal proteins such as NUDE-1, synapsin1, DARPP32 and the Munc18/Syntaxin1A complex. The evidence also suggests that aberrant Cdk5 activity induced by the conversion of p35 to p25 plays a role in the pathogenesis of neurodegenerative diseases such as Alzheimer's disease (AD), amyotrophic lateral sclerosis (ALS) and Niemann's Pick type-C disease (NPD). Abnormal hyperphosphorylation of tau after Aβ1-42 treatment destabilizes microtubules, contributing to neurite degeneration and the formation of paired helical filaments (PHFs) containing neurofibrillary tangles (NFTs), one of the principal lesions of AD. It has further been found that cdk5 is necessary for correct neuronal development
The p35 protein which acts as a regulator of CDK5 activity has recently been identified as a client protein for HSP90 and therefore the activity of CDK5 can be regulated by changes in the level and activity of HSP90. Thus inhibition of HSP90 can lead to loss of p35, an inhibition of CDK5, a reduction of phosphorylated tau protein in susceptible individuals and may bring benefit to sufferers of Alzheimers Disease.
Additionally inhibition of HSP90 using known agents has been shown to reduce the accumulation of tau protein aggregates in cellular systems in vitro. (Dickey et al Curr Alzheimer Res. 2005 April; 2(2):231-8).
Cdk5 has also been shown to have a role in mediating pain signalling. Both Cdk5 and p35 have been shown to be expressed in nociceptive neurons. In p35 knockout mice, which show substantially reduced Cdk5 activity, the response to painful thermal stimuli is delayed (Pareek, T. K., et al., Proceedings of the National Academy of Sciences., 103:791-796 (2006). Additionally administration of the cyclin-dependent kinase 5 (Cdk5) inhibitor roscovitine has been shown to attenuate formalin-induced nociceptive responses in rats (Wang, Cheng-haung, et al., Acta Pharmacologica Sinica., 26:46-50 (2005). Activation of calpain is calcium dependent and is known to affected by activation of the NMDA receptor calcium channel (Amadoro, G; Proceedings of the National Academy of Sciences of the United States of America, 103, 2892-2897 (2006)). NMDA receptor antagonists are know to be clinically effective against neuropathic pain conditions (Christoph, T; et al., Neuropharmacology, 51, 12-17 (2006)). This efficacy may be linked to the effect of NMDA receptor related calcium influx on calpain activity and its subsequent effect on the activity of Cdk5. As such compounds modulating Cdk5 activity are expected to be useful for the treatment or prevention of pain and thus modulation of the CDK5 regulator p35 by HSP90 inhibition could lead to inhibition of CDK5.
It is desirable to have an agent for the palliative treatment of pain, i.e. the direct relief of pain in addition to the relief of pain as the result of amelioration of the underlying disease or medical condition, which is the cause of the pain.
Various Cdk's (especially Cdk's 4, 5 & 6) have been shown to be involved with or mediate neuronal death following hypoxic or ischemic insult (Rashidan, J.; et al.; Proceedings of the National Academy of Sciences., 102:14080-14085 (2005). Furthermore the Cdk inhibitor flavopiridol has been shown to significantly reduce neuronal death in a rat model of focal cerebral ischemia (Osuga, H.; et al.; Proceedings of the National Academy of Sciences., 97:10254-10259 (2000). Cdk5 inhibitors have been shown to have protective effects in both necrotic and apoptotic paradigms of neuronal cell death (Weishaupt, J.; et al.; Molecular and Cellular Neuroscience., 24:489-502 (2003). Stroke is a cerebrovascular event, which occurs when the normal bloodflow to the brain is disrupted, and the brain receives too much or too little blood. Stroke is one of the leading causes of death worldwide, and is also one of the most common causes of neurologic disability.
Ischemic stroke, which is the most common type of stroke, results from insufficient cerebral circulation of blood caused by obstruction of the inflow of arterial blood. Normally, adequate cerebral blood supply is ensured by a system of arteries within the brain. However, various disorders, including, but not limited to inflammation and atherosclerosis, can cause a thrombus, i.e., a blood clot that forms in a blood vessel. The thrombus may interrupt arterial blood flow, causing brain ischemia and consequent neurologic symptoms. Ischemic stroke may also be caused by the lodging of an embolus (an air bubble) from the heart in an intracranial vessel, causing decreased perfusion pressure or increased blood viscosity with inadequate cerebral blood flow. An embolus may be caused by various disorders, including, but not limited to atrial fibrillation and atherosclerosis.
A second type of stroke, hemorrhagic stroke, involves a hemorrhage or rupture of an artery leading to the brain. Hemorrhagic stroke results in bleeding into brain tissue, including the epidural, subdural, or subarachnoid space of the brain. A hemorrhagic stroke typically results from the rupture of an arteriosclerotic vessel that has been exposed to arterial hypertension or to thrombosis.
One opportunity for intervention in stroke is the prevention or reduction of risk of stroke in patients at risk for stroke. There are many known risk factors for stroke, including, but not limited to vascular inflammation, atherosclerosis, arterial hypertension, diabetes, hyperlipidemia and atrial fibrillation. At risk patients have been treated with agents to control blood pressure or manage blood lipid level, and have been treated with antiplatelet agents (such as clopidrogel) and anticoagulants. A second opportunity is the treatment of acute stroke. However, current pharmacologic therapies for treating acute stroke are limited to restoring blood flow within a narrow therapeutic time window of less than three hours after stroke. There remains a need for agents which are effective within a longer therapeutic time window. Another opportunity is recovery or restoration after the acute stroke period, i.e. the reduction or prevention of secondary cell damage in the penumbra. There remains a need for agents which are effective in reducing or preventing secondary cell damage after stroke.
It would be desirable to obtain a single pharmaceutical agent which can be used in more than one of the above-mentioned opportunities for treating stroke. Such an agent may be administered to patients at risk for stroke, and also may be administered to patients suffering from acute stroke, or patients undergoing treatment for recovery or restoration after the acute stroke period. Such an agent may also target more than one distinct mechanism in the biochemical cascade of stroke.
HSP90 Inhibitors and the Treatment of Hepatitis C and Other Viral Diseases
Infection of a host cell with viral RNA/DNA results in a substantial redirection of cellular protein sysnthesis towards key viral proteins encoded by the viral nucleic acid. The increased protein synthetic burden places a stress on the cell as a consequence of increased demand for energy and synthetic precursers. Upregulation of heat shock proteins is frequently a consequence of viral infection at least in part due to this stress. One function of the HSP induction may be to assist in the stabilization and folding of the high levels of ‘foreign’ protein generated in preparation for virus replication. In particular recent work has suggested that HSP90 is required for stable production of functional NS2/3 protease in Hepatitis C (HCV) replicon infected cells. HSP 90 inhibitors have also been demonstrated to block viral replication in in vitro systems. (Nagkagawa, S, Umehara T, Matsuda C, et al Biochem. Biophys. Res Commun. 353 (2007) 882-888; Waxman L, Witney, M et al PNAS 98 (2001) 13931-13935).
Heat Shock Proteins and Antitumour Drug Resistance
It has long been recognized that the native tertiary conformation of any given polypeptide is determined by its primary (amino acid) sequence. However, as explained above, it is now clear that the proper folding of many proteins in vivo requires the assistance of heat-shock proteins (Hsps) acting as molecular chaperones. While this chaperone function is important to normal cellular function under all conditions, it becomes crucial in cells which are stressed (for example by heat, hypoxia or acidosis).
Such conditions typically prevail in tumour cells, which exist in a hostile host environment. The upregulation of Hsps often seen in such cells is therefore likely to represent a mechanism by which malignant cells maintain the integrity of their proteomes under conditions which compromise protein folding. Thus, modulators or inhibitors of stress proteins in general (and Hsp90 in particular) represent a class of chemotherapeutics with the unique ability to inhibit multiple aberrant signalling pathways simultaneously. They can therefore exert antitumour effects whilst eliminating (or reducing the incidence of) resistance relative to other treatment paradigms.
Moreover, therapeutic anticancer interventions of all types necessarily increase the stresses imposed on the target tumour cells. In mitigating the deleterious effects of such stresses, Hsps are directly implicated in resisting the effects of cancer drugs and treatment regimens. Thus, modulators or inhibitors of stress protein function in general (and Hsp90 in particular) represent a class of chemotherapeutics with the potential for: (i) sensitizing malignant cells to anticancer drugs and/or treatments; (ii) alleviating or reducing the incidence of resistance to anticancer drugs and/or treatments; (iii) reversing resistance to anticancer drugs and/or treatments; (iv) potentiating the activity of anticancer drugs and/or treatments; (v) delaying or preventing the onset of resistance to anticancer drugs and/or treatments.
Pro-Drugs
Pro-drugs are generally recognised as being chemical compounds that have little or no pharmacological activity themselves but which undergo biotransformation to a therapeutically active metabolite in vivo (see for example Bernard Testa, Biochemical Pharmacology, 68 (2004), 2097-2106, and “Design of Prodrugs” (Bundgaard H. ed.) 1985 Elsevier Science Publishers B. V. (Biomedical Division) and Rautio et al., Nature Reviews (Drug Discovery), Volume 7, March 2008, 255 to 270.
Pro-drugs are used for a variety of reasons. For example, they may be used inter alia to:                impart better solubility to an otherwise insoluble or poorly soluble drug        improve chemical stability        improve the organoleptic properties of a drug        alter the pharmacokinetics of a drug        reduce pre-systemic (first pass) metabolism        reduce the extent of conjugation of a drug        improve oral absorption        provide selective targeting of a drug        provide in situ activation of a cytotoxic agent        
Whereas as many as 5-7% of drugs approved worldwide can be classified as prodrugs (Rautio (2008)), the development of pro-drugs of drugs containing phenolic hydroxyl groups has proved somewhat problematic.
Various derivatives of hydroxyl groups have been proposed and/or investigated (see Rautio idem) but with varying results.
In the case of the terbutaline prodrug (bambuterol), the two phenolic hydroxy groups of terbutaline have been derivatised to give dimethylcarbamoyloxy groups which are slowly converted back to the hydroxyl groups in vivo to regenerate terbutaline.
However, elsewhere in the literature, it has been found that many simple dialkylcarbamate derivatives of hydroxyl compounds are too stable and too resistant to hydrolysis to function as pro-drugs (Igarashi et al., Chem. Pharm. Bull., 55(2), 328-333 (2007)—see in particular page 329 column 2).
Simple dialkylcarbamate prodrugs have also been associated with toxic side effects (Thorberg et al., J. Med. Chem., 1987, 30, No. 11, 2008-2012—see in particular page 2010. column 2). Thorberg et al found that arylcarbamate derivatives did not give rise to the same toxic effects.
Monoalkylcarbamate derivatives of phenolic hydroxyl compounds have also been investigated as potential prodrugs. Igarashi et al., (idem) found that the monoethylcarbamoyl derivative of a phenolic capillarisin analogue gave good plasma levels of the parent phenolic compound but that other monosubstituted carbamates were too readily hydrolysed or metabolised to be suitable as prodrugs. Furthermore, like dialkylcarbamates, monoalkylcarbamates have also been associated with toxic side effects (Thorberg et al., idem page 2010 column 2).
Ester and ether derivatives of the phenolic dopamine autoreceptor agonist (−)-3-(hydroxyphenyl)-N-propyl-piperidine were also investigated by Thorberg et al. as potential prodrugs but they found (see page 2010 column 1) the ether and acyl ester derivatives failed to generate the parent compound in plasma, possibly because of a lack of stability and a tendency to hydrolyse while in the digestive tract.
Thus, as indicated above, the development of prodrugs for phenolic compounds is far from straightforward and functional group derivatives that may provide useful prodrug properties in one class of compounds may be ineffective or may even give rise to toxicity problems in other classes of compounds.
WO 99/29705 (Glycomed et al) discloses a class of glycomimetic compounds having a number of possible uses including the treatment of cancer. One compound specifically disclosed in WO 99/29705 is the compound 2-(2-hydroxy-benzoyl)-1,2,3,4-tetrahydro-isoquinoline-3-carboxylic acid.
WO 2006/117669 (Pfizer) discloses a class of amide resorcinols as Hsp90 inhibitors. International application WO2006/109085 (Astex Therapeutics) discloses hydroxybenzoic acid amides as Hsp90 inhibitors.
WO 2008/044027 (Astex Therapeutics) discloses prodrugs of the compounds of WO2006/109085.
Citation or identification of any document in this application is not an admission that such document is available as prior art to the present invention.